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Notes: The Black Hole War by Leonard Susskind Observation: David Bohm writes on the concept of a holographic principle applied to the universe in his 1980 book Wholeness and the Implicate Order. Questions: what is the distinction between mass and information in a black hole vs a hologram? Synopsys of thoughts on a “holographic universe”: According to the author, two facts are paradoxical: 1) black holes exist, and “swallow” matter. 2) As established by Stephen Hawking, black holes evaporate over time. According to the author, his “holographic principle” resolves the paradox by stating that the matter apparently swallowed by the black hole is in fact information located at the edge of the universe; ie, it was never in the black hole to begin with Introduction: By the 1950s, Richard Feynman, Julian Schwinger, Sin-Itiro Tomanaga and Freeman Dyson had laid the foundation for a synthesis of Special relativity and QM called Quantum Field Theory. P. 7. So far, reconciling QM and the general theory (gravity) has been intractable. …where distances are far too small to be directly observed, nature’s smallest objects exert powerful gravitational forces on one another. p. 8. The new concept will be quantum gravity, what ever that is. QG deals with objects 100 billion billion times smaller than a proton. . 9. The First Shot: As of 1983, Steven Hawking believed that bits of information swallowed up by a black hole were irretrievably lost. He had proved that black holes eventually evaporate, leaving no trace of what has fallen in. He also postulated that the vacuum was full of virtual black holes that flashed into and out of existence so fast that we are not aware of them, and these also erased information. Susskind and Gerard ‘t Hooft did not believe Hawking, and set out to prove him wrong. P. 22 f. What’s so bad about loosing information inside a black hole? Losing information is the same a generating entropy, and generating entropy means generating heat. Susskind calculated that if Hawking was right, empty space would heat up to a thousand billion billion billion degrees in a fraction of a second. The Dark Star: A neutron star is so dense that a single teaspoon would weigh ten trillion pounds, but it is not yet a dark star; the escape velocity would be 80% of the speed of light. In 1917, astronomer Karl Schwarzschild solved the equations of general relativity and rediscovered the dark star. Schwarzschild radius: the radius of an imaginary sphere within which light will be pulled into the dark star (or black hole.) The black hole itself is a “singularity”; ie a point in space. The Schwarzschild radius is proportional to the mass. “Nothing can survive it’s [the singularity’s] infinitely powerful forces.” Einstein’s Equivalence Principle: there is no difference between the effects of gravity and the effects of acceleration. In itself this was not surprising, but the consequences are momentous: if acceleration makes the trajectory of a light ray bend, then so will gravity. The world as Hologram: “There is something crazy about string theory that I first came across in 1969, but it is so crazy that string theorists don’t even want to think about it…. We all expect that electrons, photons, and other elementary particles are at least as big as the Plank length, and possibly bigger. The problem is that the mathematics of string theory implies an absurdly violent case of quantum jitters [zero point fluctuations], with fluctuations so ferocious that the pieces of an electron would spread out to the ends of the universe. To most physicists, including string theorists, that seems so crazy that it is unthinkable….. p. 293. “String theory …places every bit of information, whether in black holes or black newsprint, at the outer edges of the universe… p.294. “Here, then, is the conclusion that ‘t Hooft and I had reached: the three dimensional world of ordinary experience-the universe filled with galaxies, stars, planets, houses, boulders and people- is a hologram, an image of reality coded to a distant two dimensional surface. This new law of physics, known as the Holographic Principle, asserts that everything inside a region of space can be described by bits of information restricted to the boundary.” P. 298. “That information is distributed throughout the volume of space seems so intuitive that it’s hard to believe it could be wrong. But the world is not voxelated [the three dimensional equivalent of a 2-D pixel], it is pixilated, and all information is stored on the boundary of space. But what boundary and what space? p. 299. “It is evident that the question of where a particular bit of information is located does not have a unique answer. “As we try to be more and more exact, especially when we account for both gravity and Quantum Mechanics, we are driven to a mathematical representation involving pixels dancing across a distant two dimensional screen and a secret code for translating the scrambled patterns into coherent three-dimensional images. P. 300. “As weird as the Holographic Principle is … it has become part of the mainstream of theoretical physics. It is no longer just a speculation about quantum gravity; it has become an everyday working tool, answering questions not only about quantum gravity but also about such prosaic things as the nuclei of atoms. “ The major difference between conventional holograms and the world as hologram is that the world hologram is quantum mechanical. The weapon that brought the [Black Hole] war to a close was largely the rigorous mathematics of String Theory. [The same theory whose rigorous mathematics predicts zero point fluctuations so ferocious that the pieces of an electron would spread out to the ends of the universe]. Weapon of Mass Deduction By itself, the holographic principle was not enough to win the black hole war. It was too imprecise, and lacked a firm mathematical foundation. What is it that takes fringe idea and tips the scale in its favor? An experimental or mathematical result. “No ones knows whether String Theory is right for our world… ST is based on QM; it describes a system of elementary particles similar to those in our universe. It assumes all material objects interact through gravitational forces, and has black holes. We do have impressive evidence that string theory is a mathematically consistent theory of some world….[we] calculate, or prove mathematically, whether or not information is lost in black holes in that world. Source books for details on string theory: Susskind: The Cosmic Landscape, Brian Green: The Elegant Universe, Lisa Randall: Warped Passages. String theory has two applications in modern physics. Started out as Hadronic string theory (hadrons: include protons, neutrons, mesons, and gueballs) That hadrons are string like objects, called QCD strings, described by the mathematics of string theory, is an accepted fact. Fundamental strings; the ones associated with gravity and the Plank length, much much smaller than hadrons, are the ones that have created all the controversy. Hadronic string theory How can we tell if particles are “elementary”? One way might be to smash two particles together and see what comes out. A better way is to see if you can spin the object. “The ability to set an object into rotation, or to start its shape oscillating, is a sure sign that it is made of smaller parts, that can move relative to one another.” Angular momentum is a combination of velocity, size and mass: ½ massX(velocity squared). If energy is plotted vs angular momentum of a basketball, we have a quadratic shaped curve which abruptly stops when the ball is destroyed by centrifugal force. For molecules atoms and nuclei, the shape of the curve is similar, but energy is added in discreet quanta. The graph of energy vs angular momentum is a sequence of separate points. [The Italian physicist Tullio Regge was the first to study the properties of such plots, which are called Regge trajectories.] No one has been able to spin electrons (ie, give them more angular momentum) Protons and neutrons together are called nucleons. Nucleons are complex. A plot of energy vs angular momentum of a single nucleon turns out to be not a quadratic, but a straight line, and does not seem to have a breaking point. So a nucleon is made of parts, but whatever holds the parts together is much more tenacious than the forces holding the nucleus together. The nucleon stretches out as it rotates, but not in a disk; rather it stretches out into a long thin string-like object [that appears to tumble about an axis perpendicular to the string. Gluon is the sticky material that forms long strings and keeps quarks from flying apart. Mesons consist of one quark and one antiquark joined by a sticky string, and can vibrate and move in a variety of ways. Nucleons contain 3 quarks, each attached to a string, and the three strings are joined at then center. They can also twirl and vibrate. Rapid movement of a hadron adds energy, stretches the string out and increases its mass. Quarkless particles exist, forming a closed loop, called glueballs, like a rubber band. Gluons have two ends, positive and negative, almost like they are tiny bar magnets. The mathematical theory of quarks and gluons is called quantum chromodynamics. (QCD) If a quark in a meson is hit with a large force, the quark will separate from its partner, but the gluons holding the two quarks together will clone themselves, stretching longer. The string theory of nucleons, mesons, and glueballs is well established as a part of the standard theory of hadrons. Quarks seem to be as small and as elementary as electrons. Many differet types of quarks, whose internal workings are not understood. All of these different kinds of quarks come in three different categories called “colors”: “red”, “blue”, and “green”. Permutation of “red” “blue” and “green” on each end give Nine kinds of gluons. The whole thing is too messy. Why keep working on particle physics then? The very messyness must tell us something about nature. Fundamental string theory The most important Fundamental string is the graviton, the quanta of the gravitational field. Electrical and gravitational forces are similar: both follow the inverse square law, the coulomb force between two particles is proportional to their charges; the gravitational force between two objects is proportional to the product of their masses. Both types of forces can create waves. Gravitational waves are to mass what electromagnetic waves are to electric charge. The energy of electromagnetic waves is from the quanta of photons. By analogy, the energy of gravity waves must come fm the quanta of gravitons. The existence of gravitons is however experimentally unverified. EM radiation is explained in quantum field theory by a “vertex” diagram in which a charged particle, for example an electron, emits a photon. Since all particles are effected by gravity, all particles must be able to emit gravitons. Including gravitons in Fenyman diagrams causes mathematical problems. For almost 50 years, physicists have tried unsuccessfully to make sense of a Quantum Field Theory of gravitons. The trouble with Quantum Field Theory is that no matter how finely you divide space, you can always subdivide it further. Infinitely divisible space is called a “continuum”. The infinite potential to add ever smaller structure to the Feynman diagram is a disquieting consequence of the space time continuum of Quantum field theory. It is even more out of control for a Quantum field theory of gravity. Gravity is geometry, and as the piece of space becomes more infinitesimal, the more violent fluctuations would be. Physicists suppose that space is not a true continuum, that if you keep subdividing, you will discover an indivisible nugget of space. Feynman diagrams, even those involving gravitons, make sense as long as you do not add structures smaller than the Plank length. The expectation was that you would have an indivisible granular structure at the Plank length. But that was before the discovery of the holographic principle. Replacing continuous space with an array of granules gives a misleading holographic picture. String theory is a holographic theory describing a pixilated space (?) The great tensile strength of fundamental strings make them hard to stretch very far, so the size of a fundamental string is close to the Plank length. Why aren’t accelerators built to settle the question of whether particles are vibrating fundamental strings? The energy needed to excite hadrons is modest; The energy needed to excite fundamental strings is enormous. No accelerator could be built to do it. For this reason string theory will remain an experimentally unproven theory. As you add energy to a string, it lengthens; if you add enough energy, it would become a violently jittering ball of yarn. Black holes are enormously large twisted “monster strings”. The subtle math of string theory goes haywire unless more dimensions are added. Six more dimensions of space are needed to keep the equations from breaking down. With nine directions to move in String theory is mathematically consistent. The idea is that the extra dimensions of space are wrapped up in very small knots that are too small to be detected. All modern theories of elementary particles make use of some form of extra dimensions. There is the Faraday-Maxwell theory of fields. Richard Feynman showed up with a quantum theory of force, in which a charged particle is a juggler of photons. Both are correct. It is true that string theory is not yet a full blown theory, but it is far and away our best math guide to the ultimate principles of quantum gravity. Alice’s Airplane Black Hole Complementarity: depending on the state of motion of the observer, an atom might remain a tiny microscopic object, or it might spread out over the entire horizon of an enormous black hole. Author posits “Alice’s propeller”: a fractal like propeller that is built from smaller propellers, that rotate faster, being attached to the ends of each new propeller blade. In the 1970s, black hole theorists, especially Willim Unruh, showed that near a black hole horizon, thermal energy and ZPE get mixed up in odd ways. Fluctuations that appear benign (ZPE fluctuations) to someone falling through a black hole become dangerous thermal fluctuations to someone poised just above the horizon of the black hole. Quantum Field Theory begins by postulating particles that are so small they can be regarded as points in space. But that picture quickly breaks down. These particles quickly surround themselves with other particles that come and go at a tremendous pace. These new comers-and-goers are themselves surrounded by even more rapidly appearing and disappearing particles. String Theory: a camera with a slow shutter speed will show an elementary particle as a point. Speed up the shutter to where it stays open for a bit longer than the Plank unit of time, and we will see the particle is really a string (loop). Increasing the shutter speed more and more will cause the string to spread out into more rapidly fluctuating loops and squiggles. If we watch a string like particle fall into a black hole, because vibrations appear to be slowed, we see more of the oscillating structure of the string, until it grows and spreads over the entire horizon of the black hole. If we fall along side the particle, it remains to appear a point. Quantum Field Theory and String Theory share the property that things appear to change as shutter speed increases. “But in Quantum Field Theory the objects do not grow. Instead they appear to break down into progressively smaller objects. …Things inside things inside things” “String Theory is different…as things slow down, more and more string comes into view” Author posits that for Stephen Hawking, Quantum Field Theory, with it’s point particles, was the be-all and end-all of microscopic physics…and this was Hawking’s blind spot. [but what if we include in our high speed photograph an image of the Quantum Field Theory particle as it is engaging with the other particles that come and go at a tremendous pace, and those by even more rapidly appearing and disappearing particles? This is more analogous to Alice’s propeller, and has the result that as we take faster images, we will see more of the particles and virtual particles connected. What does QFT really look at, only the slowest single point?] Counting Black Holes Stephen Hawking calculated the entropy of a black hole equals the horizon area in Plank units. Hawking and others also believed that black hole entropy had nothing to do with counting quantum states. Why? Because Hawking argued that one could keep throwing more and more information into a black hole without any information leaking back out. So the question is whether or not the amount of information going into a black hole is limited or not. “Does black hole entropy really count the possible configurations of a black hole?” String theorists gave a firm quantum mechanical basis for BekensteinKawking entropy, that left no room for information loss. A spectrum of elementary particle mass would have as the lightest particles photons and gravitons with no mass. Increasing in mass would be electrons, and hadron components, then “superpartners” grand unification particles, and finally string excitations, which are just excite states of ordinary particles. the heaviest mass would be the Plank mass. ‘T Hooft’s conjectures that the spectrum of particles continues on to indefinitely large mass in the form of black holes, which have certain discrete masses. There is good reason to accept this conjecture. Strings have entropy. The presence of a photon represents one single bit of information. As more energy is pumped into the photon, it becomes a string with thermal jitters. The tangle of the string is too complicated to describe in detail, but a rough description is possible. This is the “hidden information” that gives the agitated string its entropy. As a ball of string shrinks and morphs into a black hole, the mass and energy may change but the entropy remains the same (Adiabatic theorem), with entropy proportional to square of mass. The pictre of a black hole horizon that emerges is a tangle of string flattened out on the horizon by gravity. String bits may break off, so the black hole looses a bit of energy. That is how string theory explains Hawking radiation. Authors ideas of Black Hole Complementarity, Alices airplane, and strings morphing back and forth into black holes more less accepted by Rutgers physicists. It was understood that if electrons were dropped into a black hole, the horizon could become electrically charged. Physicist Cumrun Vafa pointed out that there is a very special kind of charged black hole that is in perfect balance between gravitational attraction and electrical repulsion. It has entropy, but no heat or temperature. Such black holes are called Extremal. Vafa argued that if we knew how to make such a black hole model in string theory, it could be studied in great detail. In 1993, string theory did not have the elements to model such an extremal black hole. Brane is a string theory term, coming from membrane, a 2-D surface that can stretch. Joseph Polchinski’s 1995 discovery of “D-branes” would have profound repercussions on everything from black holes to nuclear physics (the string theory versions?) 0-branes are points: zero dimension 1-branes string: 1-D 2-branes membranes 2-D 3-branes volume If space has six compact dimensions, so there are 9 dimensions, space can hold up to and including 9-branes A D brane means a fundamental string can end on it. D0, D1, D2, D3. In 1995, Joe realized that D-branes filled an enormous math hole in string theory. There existence was necessary to complete a growing web of string theory logic and math. Dbranes were the necessary modeling tool needed to build an extremal black hole. Several groups of string theorists started using the D-branes and confirmed each others work. Extremal black holes are at absolute zero, which means they do not evaporate. They construct “almost extremal” black holes which shed excess energy without evaporating, and return to the extremal state. Callan and Maldacena were able to use string theory to compute the rate at which”almost extremal” black holes evaporate. They were able to make detailed calculations of the evaporation rate. Their results agreed exactly with Hawkings 20 year old method, except they had used only the conventional methods of quantum mechanics. But QM forbids information loss, so there is no possibility that information could be lost during the evaporation process. The fact that black hole entropy can be accounted for by the information stored in string wiggles went strongly against the views of many relativists, including Hawking. ADS space William de Sitter was a Dutch physicist, mathematician, and astronomer who discovered the 4-D solution of Einstein’s equation. Mathematically, de Sitter space is an exponentially expanding universe. It is a curved space time continuum with positive curvature, meaning that the angles of a triangle add up to more than 180 degrees. Anti di Sitter space means the curvature is of space is negative; ie the sum of the angles of a triangle are less than 180 degrees. This space has many properties of a spherical box, but a box that cannot be swallowed by a black hole, because the spherical limit of the space exerts a powerful repulsion. ( cosmologists have found that our universe is expanding at an accelerated rate. This exponential expansion is thought to be due to a “cosmological constant” or what the popular press calls “dark energy”) The ADS space that Banados, Teitelboim and Zanelli worked with was 2D + time. A circular two dimensional slice of ADS space is also curved. To draw it on a flat surface it must be distorted. Distorting it into a plane has an “anti-Mercator” effect, making things near the edge too small. Esher’s drawing Circle Limit IV is “map” of negatively curved space that shows what a slice of ADS looks like. The angels and devils go on endlessly, becoming smaller and smaller as they fade into an infinite fractal edge. We can think of time as the direction perpendicular to the circular 2D surface of the ADS space, so the ADS time continuum is like a 3-D cylinder. Time is also warped is ADS. The more distant form the center of the circle, the faster time goes. The space –time curvature in ADS creates a gravitational field that pulls objects to the center, even if there is nothing there. One property of this ADS gravitational field is that if a mass were displaced toward the boundary, it would be pulled back to the center. If mass is added, it will accumulate in the center and be acted on by traditional gravity. If enough mass is added (if the ADS space has enough area) a black hole would be formed, trapped in the ADS box. This is called a de Sitter black hole, which could not evaporate. If one more dimension were added to Escher’s drawing, it is not hard to see how it would look in 3-D. A revised definition of the holographic principle becomes “Everything inside a box with impenetrable walls (an ADS space) can be described by bits of information stored in pixels on the walls” [ But the author has altered the definition of an ADS space: the definition is that objects get smaller and smaller, and time goes faster as we approach the outer boundary, which implys that we are not talking about bits at the walls, but infinitely small objects. If we consider space as a continuum, then as an object, say the angel in Escher’s art, can remain a distinct object indefinitely, and not become “bits” at all. Are they the same? Another difference is that the 2-D outer surface of the hologram contains distributed information the emplicate order, whereas the information on the ADS wall, even if it is pixels, is not distributed, but has a one to one direct correspondence with the information inside the ADS space. ] Maldacena’s amazing discovery Maldacena argued that two mathematical worlds that seem totally dissimilar are in fact exactly the same. One world has four dimensions of space and one of time, the other has three dimensions of space and one of time….the route to discovery was a convoluted path that wandered through extreamal black holes and D-branes and ended with a confirmation of the holographic principle. The two worlds are called “dual” in the sense that photons have a wave particle duality. “Everything that takes place in the interior of anti de Sitter space ‘is a hologram, an image coded on a distant two-dimensional surface’. A three-dimensional world with gravity is equivalent to a two-dimensional quantum hologram on the boundary of space” Something called “matrix theory” was the first example of a mathematical correspondence confirming the “Holographic Principle.” 23: Nuclear Physics? You’re kidding! Skeptics will point out that everything I have told you about the quantum properties of black holes, from entropy temperature and Hawking Radiation to Black Hole complementarity and the holographic principle, is pure theory without a shred of evidence to support it. That may be true. P. 422 However, a totally unexpected connection has recently turned up between black holes, quantum gravity, the holographic principle, and experimental nuclear physics which may support the above described theory. P. 422 The mathematics of the thoroughly established hadron physics turns out to be almost the same as the mathematics of string theory, despite the fact that hadrons are 10 20th larger than fundamental strings. If ordinary sub-nuclear particles are really similar to fundamental strings, why not test the ideas of string theory in well established nuclear physics? P. 423 Until recently it has not been possible to test the nuclear analog of Black Hole physics, but that is changing. P. 423 Nuclear physics take place on an immensely larger scale than fundamental string physics, so it takes much less energy concentrated greatly larger volume. (chapter 16) p. 425 When the numbers are plugged in, something very similar to slow motion magnified black holes should form when ordinary nuclei collide in the RHIC (Brookhaven National Lab Relativistic Heavy Ion Collider.) p. 425 To understand in what sense black holes are created by RHIC, we have to return to Juan Maldacena’s discovery: two different mathematical theories are really the same; “dual” to each other. One theory is string theory, with gravitons and black holes in a 4+1 dimensional ADS space. The holographic principle states that everything that takes place in the ADS must be completely describable by a math theory in one less dimension; ie, in 3-D space. The holographic dual is mathematically quite similar to QCD p. 425. ADS - QCD Hypothesizes hadrons are in a “q-space” bounded by a “UV-brane” and an “IR-brane”. The UV-brane is similar to the bounding surface of the ADS space where particles shrink and speed up. The IR-brane is the boundary where particles expand and slow down; where the hadrons live. P. 426-427. There are two views of the similarity between controversial fundamental string theory and established hadron (nuclear physics) theory: The conservative view is that it is coincidental; the other less conservative view is that nuclear strings really are the same objects as fundamental strings. Again theorizing that when particles are near the UVbrane they are small and energetic, and when they are near the IR brane they are big and slow. For example, a closed string (loop) near the UV brane would be a graviton, and near the IR brane would be a glueball. [Is there a similar pattern with other fundamental and nuclear particles? There seem to be a lot more nuclear particles, and only one fundamental particle: a graviton] When two gravitons collide (near the UV-brane) a small black hole may be formed. When two nuclei collide (near the IR-brane): in four dimensions, a low-energy black hole forms in three dimensions, we get a splash of quarks and gluons. P. 430. What is actually observed in the collision of two nuclei is not a gas, but a blob of fluid with surprising flow properties which resemble the horizon of a black hole. This fluid from the collision of nuclei and the horizon of the black hole both have a viscosity much lower than superfluid helium. P. 431 Humility p 433 f If you look out far enough in an expanding universe, you will come to a point where the galaxies are moving away from you at the speed of light. [nothing can travel at or faster than the speed of light ?] in every direction we look, galaxies are passing the point at which they are moving away from us faster than light can travel[?] We are surrounded by a cosmic horizon. And no signal can reach us from beyond that horizon. Far out, at about 15 billion light years, our cosmic horizon is swallowing galaxies, as if we all live in our own inside out black hole. There is no indication that galaxies thin out or come to an end at the horizon. Quantum gravity is about densely packed information and entropy in a black hole. Epilogue As of 2002, Roger Penrose still argued that information is lost in the black hole evaporation. His arguments were the same as Hawking’s 26 years earlier. He argues that the holographic principle and Maldacena’s work were based on a series of misconceptions. He doesn’t go along with the idea of dimension reduction: “How can physics in more dimensions be the same as physics in fewer dimensions?” In 2004, Hawking changed his mind; information does leak out of black holes and ends up in the evaporation products.